Osu!! Karate Bu takes place at Kansai Fifth Technical High School, a school known for its delinquents, even for Osaka. Its karate club is known widely as the toughest thugs of all. First-year Matsushita Tadashi joins the karate club to stop people from picking on him, but ends up with more than he bargained for. The series soon focuses on the captain, Takagi Yoshiyuki, the strongest gangster in Osaka, and his struggle to protect the karate club and protect the city of Osaka.